I couldn't make it static, assume not important anyways?

I'd say your version is better than a static verison.

The fully static version would be:

PhabricatorEditEngine::generateFullKeyForFormFromTheseConstants(
  ManiphestTaskEditEngine::ENGINEKEY,
  PhabricatorEditEngine::EDITENGINECONFIG_DEFUAULT);

But I think your version is cleaner.

You can technically do your version as a static method, like this:

public static function generateFullFormKey() {
  return static::ENGINEKEY.'/'.self::EDITENGINECONFIG_DEFAULT;
}

...using static instead of self, but this is a feature called "Late Static Binding" which was not introduced until PHP 5.3, so we can't currently use it in this codebase.